Like "2001: A Space Odyssey," Jake Kasdan's "Sex Tape" is a grim cautionary fable about the evils of technology, in this case pitting its desperate protagonists against an unseen force people refer to as "the cloud."

Unlike "2001," it's also a stupid, strenuous sex farce starring Cameron Diaz, Diaz's dorsal-view body double and Jason Segel as an LA couple (she's a mommy blogger, he's in radio) with two kids, an increasingly groggy romantic life and a provocative solution to their current coital slump: Make a sex tape!

The results, a three-hour epic re-enacting of the entire "Joy of Sex" reference book, wind up sliding onto a batch of iPads that Jay (Segel, uncharacteristically forcing every joke, such as they are) has recently given away for reasons very, very poorly set up by the script. It's "uh-oh!" time when Annie, played by Diaz, hands her own porn-infected iPad to a rich Brentwood CEO (Rob Lowe, who had his own sex-tape issues once upon a time) interested in buying her blog for a handsome fee.

Rob Corddry and Ellie Kemper are the central couple's married friends, aiding Jay and Annie in their frantic retrieval of the iPads synced up (again, a lot of horsy unconvincing explanation here) with Jay's own iPad, the one we keep hearing about, with all its fantastic high-res clarity. "Sex Tape" is so in the bag for Apple and its products, it's practically a product-placement love story.

Why does this movie flail when recent R-rated commodities such as "Neighbors" (directed by Nicholas Stoller, one of the writers on "Sex Tape" along with Segel and Kate Angelo) have found a way to deal with the challenges of marriage and parenthood and temptation in a clever way? Partly it's because "Sex Tape" settles for violence when violent slapstick, a lot harder to finesse, was the implicit goal of the picture.

The centerpiece sequence has Annie doing cocaine with the weaselly Lowe character, while Jay battles the CEO's murderous guard dog elsewhere in the mansion. Director Kasdan, a middling talent at best with this sort of material, chops the action into clunky, bug-eyed close-ups or flatly staged medium shots. Early on there's a long bit where Jay, nervous about the prospect of having sex with his wife after a little too much time has elapsed, has trouble untying Annie's double-knotted roller skates. You wait for the scene to get going or pay off or something. Instead, it's just awkward. The whole movie's like that.

If "Sex Tape" ends up making money, it'll be strictly on the backs of its valiant performers, who surely sensed the weakness of the script and decided to go at it like a job, like turning an acting trick, finesse and pacing be damned. Six years ago Kevin Smith's "Zack and Miri Make a Porno" was funnier, sharper, and more interesting in its central relationship. But that was in the pre-cloud days, when dinosaurs ruled the Earth, before movies like "Sex Tape" took for granted (or pushed the idea) that everybody's into porn these days, all the time, like breathing. Everybody. All the time. Segel and Diaz have their comic charm, but it's stretched thin here to the breaking point.